The present invention relates to circuit breakers and, more particularly, to cassette molded case circuit breakers.
Circuit breakers are installed in electrical distribution circuits to provide protection against high currents produced by various overcurrent conditions such as short-circuits, ground faults, overloads, etc. Circuit breakers typically employ one or more pairs of electrical contacts, an operating mechanism mechanically coupled to at least one of the contacts, and a trip unit that senses current or other electrical condition in the electrical distribution circuit and unlatches the operating mechanism to separate the pairs of contacts upon sensing an overcurrent condition.
Molded case circuit breakers employ a molded, electrically insulative case in which the various components of the circuit breaker are housed. Cassette type molded case circuit breakers have a number of cassettes disposed in the molded case, with the number of cassettes being equal to the number of poles (phases of current) in a multi-pole electrical distribution circuit. Each cassette includes a molded, insulative housing in which the one or more pairs of electrical contacts for the pole are housed. One contact in each contact pair is mounted on a contact arm, which may be supported within the cassette by a rotor. The rotor is mechanically coupled to the operating mechanism, which acts on the rotor to pivot the contact arm within the cassette for opening and closing the contact pairs.
Underwriters Laboratory (UL) 489 standard, entitled “Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit Breaker Enclosures”, sets forth various tests for molded case circuit breakers. One such test requires that the circuit breaker pass a dielectric test after the breaker is subjected to seven short circuit tests in which the circuit breaker is expected to successfully clear the fault at the rated voltage of the circuit breaker. The seven short circuit tests cause the circuit breaker to become highly contaminated, internally, with electrically conductive carbon particles. A dielectric test is then conducted between poles of opposite polarity and ground at a voltage level of 1000 volts plus two times the rating of the breaker. For example, the dielectric test for a circuit breaker rated at 600 volts would be: 2(600 volts)+1000 volts=2200 volts. The circuit breaker must withstand this voltage for one minute without breakdown.
To ensure that a cassette type circuit breaker passes the UL 489 dielectric test, the spacing between the cassettes is increased and/or the wall thickness of the cassette housings are increased to provide an adequate amount of insulation and isolation (as specified in UL 489) between phases in the circuit breaker. However, the ability to increase the pole spacing and/or wall thickness of the cassette housing can conflict with the desire to provide a compact circuit breaker. That is, increasing the pole spacing and/or the wall thickness of the cassettes to improve the dielectric integrity of the circuit breaker will also require that the overall size of the circuit breaker be increased. It is, therefore, desired to provide a compact circuit breaker meeting the requirements of the UL 489 dielectric test.